How to get Xbox Full Screen Experience on any Windows 11 PC — even if you don't have the option in settings

Anyone can now turn their Windows 11 PC into a (sort of) games console with Xbox Full Screen Experience! Here's how to do it.
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Xbox app full screen experience leaked, can run on handheld PCs with Windows 11 25H2

TL;DR: Microsoft's new Xbox Fullscreen Experience app, designed for handheld Windows 11 25H2 gaming PCs, optimizes system resources by disabling non-gaming functions, freeing up about 1GB of RAM.